Class 7 Science Chapter 3 Notes Heat
→ Celsius scale : Celsius scale or centigrade scale is the unit used for measurement of temperature. It is a meter scale of temperature.
→ Conduction : The process by which heat is transferred from the hotter end to the colder end of an solid object is known as conduction.
→ Conductor : The material which allow heat to pass through them easily is called conductors.
→ Convection : The process of transferring from hotter region to cooler region by actual movement of particles is called convection.
→ Insulator : The material which do not allow heat to pass through them easily is called insulator.
→ Land breeze : The process of flowing of air currents from cooler land towards the warmer sea is called land breeze.
→ Radiation : The process of transmission of heat without any contact between the source and the object is called radiation.
→ Sea breeze : The flow of cool air from the sea towards the land to replace the hot air on land is called sea breeze.
→ Temperature : The reliable measure of the hotness of an object is called temperature.
→ Thermometer : A device used to measure the temperature is called thermometer.
→ Heat is a form of energy. It flows from a region of higher temperature to a region of lower temperature.
→ Heat can be converted into other forms of energy.
- It is measured in joule.
- It can change the state of matter.
- Matter expands on heating.
→ Temperature is a measure of degree of hotness or coldness of an object.
→ Temperature is measured by a device known as thermometer.
→ The science of measurement of temperature is known as thermometry.
→ Temperature is usually measured in degree celsius (°C). For scientific work, kelvin temperature scale is used.
Formulae used for conversion
| Scale | Formula |
| Celsius | “CK 273.15 °C = (°F – 32) x 5/9 |
| Kelvin | K = °C + 273.15 K = (°F – 32) x 5/9 + 273.15 |
| Fahrenheit | °F = °C x 9/5 + 32 |
→ Thermometer works on the principle of expansion of liquids on heating to measure temperature.
→ Types of thermometers :
- Mercury thermometer can measure a wide range of temperatures as mercury has, high boiling point (-357°C ) and low freezing point (-39°C).
- Alcohol thermometer is safe to use and easily available. It has a low freezing point (-115°C) and therefore, can be used to measure very low temperatures.
- Clinical thermometer ranges from 35°C to 42°C . It is used for measuring human body temperature.
- Laboratory thermometer is used to measure the temperature of other objects like hot water, milk, etc. Its range varies but usually is between -10°C and 110°C.
→ Heat transfer from one body to another can take place by the process of conduction, convection and radiation.
- Conduction is that process of heat transfer in which heat is transferred from a hot body to cold body, provided that there is no gap between the two bodies. Movement of molecules does not take place.
- Convection is that process of heat transfer in which heat is transferred by the actual movement of molecules. Transfer of heat in liquids and gases takes place by convection. Sea breeze and land breeze are the examples of convection currents in gases.
- Radiation does not require any material medium for the transfer of heat. In this process, heat is directly transferred from one body to another. It is the fastest mode of transfer of heat.
→ Materials which allow heat to be conducted through them easily are called good conductors of heat for example, aluminium, iron and copper.
→ Materials which do not allow heat to be conducted through them are called insulators. For example, wood and plastic.
→ Light colours are good reflectors and poor absorbers of heat, whereas dark colours are good absorbers and poor reflectors of heat.
→ Woollen clothes keep us warm during winter as wool is a poor conductor of heat and it has air trapped in between the fibres.
→ The thermos flask minimizes transfer of heat by conduction, convection and radiation. The silver coating on the inner surface of bottle prevents heat transfer by radiation, the vacuum between its double wall prevents heat transfer by convection and the thinness of the glass wall stops heat leaving and entering the flask by conduction.